A. A. Efimov

High-efficiency synthesis of nanoparticles in a repetitive multigap spark discharge generator

We describe a method of obtaining aerosol nanoparticles in a repetitive spark discharge generator with 12 interelectrode gaps between tin electrodes, which operates at a pulse repetition frequency of 2.5 kHz. During synthesis of tin oxide nanoparticles in air, the mass productivity of the gas discharge generator reaches up to 9 g/h for primary particles with characteristic sizes within 5–10 nm and agglomerate size on the order of 50 nm.

Preparation of YSZ/Al2O3 composite coatings via electrophoretic deposition of nanopowders

Using electrophoretic deposition (EPD), we have produced YSZ individual ceramic coatings and YSZ/Al2O3 composite coatings for a wide range of applications in modern materials research. YSZ and Al2O3 nanopowders were prepared by high-energy physical dispersion techniques, namely, by a laser evaporation–condensation process and electroexplosion of wire, respectively. Stable nonaqueous suspensions for the EPD process have been prepared using YSZ and Al2O3 nanopowders with an average particle size of 11 and 22 nm, respectively. The YSZ/Al2O3 composite coating produced by sintering at 1200°C has been shown to have higher density in comparison with the YSZ individual coating produced at the same temperature. X-ray diffraction characterization showed that the YSZ/Al2O3 composite coating consisted of two crystalline phases: α-Al2O3 (corundum) (42 wt %) and cubic ZrO2〈Y2O3〉 (58 wt %). Quantitative analysis of electron micrographs of the surface of the films showed that the YSZ individual coating produced by sintering at 1200°C had a loose structure and contained pores (9%), as distinct from the composite coating, which had a dense, porefree grain structure.

Synthesis and Characterization of TiO2, Cu2O and Al2O3 Aerosol Nanoparticles Produced by the Multi-Spark Discharge Generator

The morphology, crystal structure and size of aerosol nanoparticles generated by erosion of electrodes made of different materials (titanium, copper and aluminum) in a multi-spark discharge generator were investigated. The aerosol nanoparticle synthesis was carried out in air atmosphere at a capacitor stored energy of 6 J, a repetition rate of discharge of 0.5 Hz and a gas flow velocity of 5.4 m/s. The aerosol nanoparticles were generated in the form of oxides and had various morphologies: agglomerates of primary particles of and or aggregates of primary particles of . The average size of the primary nanoparticles ranged between 6.3 and 7.4 nm for the three substances studied. The average size of the agglomerates and aggregates varied in a wide interval from 24.6 nm for to 46.1 nm for .

Filtration of nanosized particle aerosols by electret fibrous filters

Results of studying the filtration efficiency of various nanoaerosols in an air-cleaning system in which particles are charged in the integrated corona discharge unit and polymer fibers with electret properties serve as the filtration material are presented. The total filtration efficiency for titanium oxide (TiOx), diethyl-hexylsebacate (DEHS), and bovine serum albumin (BSA) aerosols and anti-mosquito smoke (AMS) is 99.9, 99.3, 98.6, and 98.7%, respectively. A significant (5–10%) drop in the filtration efficiency has been observed for DEHS and BSA aerosols and AMS in a particle size range of less than 80 nm. It has been demonstrated that the filtration efficiency can be substantially increased by activating the electrostatic particle-capture mechanism through the charging of particles in the corona discharge unit. In connection with this, the electrostatic mechanism is the dominant one for particles larger than 80 nm, while its contribution decreases in the particle size range of less than 80 nm with a decrease in particle size and a simultaneous increase in the contribution of the capture mechanism due to diffusion. It has also been found that the increase in the filter solidity by a factor of higher than 1.5 has a negligible effect on the filtration efficiency, thus making it reasonable to utilize low-solidity electret filters whose quality factor reaches the maximal values.

Preparation of alumina nanoparticle suspensions with narrow particle size distribution

Dynamic light scattering (DLS) was applied to the study of the process of the preparing deaggregated water suspensions of alumina nanopowders with specific surface areas of 20–140 m2/g. Nanopowders were prepared by the electric explosion of wire and laser evaporation and, according to electron microscopy (TEM), consisted of nonagglomerated spherical nanoparticles with lognormal size distribution. According to DLS, nonsedimenting water suspensions of alumina nanoparticles, stabilized by sodium citrate at a 5 mM concentration, contain substantial fraction of aggregates. The dynamics of the change in the mean average size of aggregates under exhaustive ultrasound treatment of suspensions with 10 g/l concentration for 1.5–4 h by two types of ultrasonic processors was studied. It was shown that the mean average size of aggregates exponentially diminishes by 1.5–2 times and the fraction of individual particles in suspension enlarges from 45 to 85%. Sequentially centrifuging the suspension at 18000 g separates the remaining aggregates and results in suspensions of individual alumina nanoparticles. Particle size distributions in these suspensions obtained by TEM and DLS are the same within experimental error.

Synthesis of Nanoparticles in a Pulsed-Periodic Gas Discharge and Their Potential Applications

Conditions for the synthesis of three types nanoparticles (SnO2, Al2O3, and Ag) with typical sizes in the range of 4 to 10 nm and a performance of 0.4 g/h are employed in a pulsed-periodic gas discharge in an atmosphere of air. Spherical Ge nanoparticles with a characteristic size of 13 nm are synthesized by these means for the first time with a performance of around 10 mg/h. The specific energy consumption in the synthesis of nanoparticles is for these materials in the range of 2000 to 5000 kW h/kg. The prospects for using tinoxide nanoparticles in sensor components and jets of silver nanoparticles for aerosol printing are discussed. The merits and demerits of the pulsed gas-discharge method among other gas-phase approaches to the synthesis of nanoparticles are analyzed for the current level of development.

Unipolar Charging of Aerosol Particles in the Size Range 75-500 nm by Needle-plate Corona Charger

A simple unipolar needle-plate corona charger (NPC) was designed, fabricated and tested on aerosol Al 2 O 3 particles in the size range 75-500 nm. The intrinsic charging efficiency, particle electrostatic losses, and extrinsic charging efficiency were investigated depending on the corona current (35-215 μA), corona polarity, and aerosol flow rate (12-250 l/min). It was found that the intrinsic charging efficiency of the NPC is growing with increase in the corona current and the particle size and decrease in the aerosol flow rate. However, the extrinsic charging efficiency is lower than intrinsic one due to the particle electrostatic losses. Although the NPC has very simple design, it provides the extrinsic charging efficiency comparable with that of unipolar corona chargers of other types having more complicated design. The maximum value of the extrinsic charging efficiency was more than 40% for particles in the size range from 98 to 210 nm.

Electrostatic Aerosol Deposition Method for the Formation of an Ensemble of Monodisperse Particles on a Substrate

We have developed an aerosol-based technique for deposition of monodisperse ensembles of spherical SiO2 nanoparticles on the surface of single-crystal silicon substrate (1 cm2) with an average surface particle density of about 2.1±0.4 particles per μm2. The obtained samples of monodisperse ensembles SiO2 nanoparticles was characterized by scanning and transmission electron microscopy. The ensemble of deposited nanoparticles is characterized by a narrow size distribution with a modal size of 26.6 nm and a full width at half maximum of 3.5 nm according to the atomic force microscopy data. We have demonstrated the use of the obtained test structure to determine the effective radius of the tip of an atomic force microscope.